Nuclear magnetic resonance (NMR) and positron emission tomography (PET) are complimentary techniques that are presently used to study in vivo anatomy and metabolism of brain and heart tissues. Correlation between tissue anatomy and functionality is currently accomplished by obtaining PET data on one scanner and NMR data on another scanner. PET and NMR images are transferred to a computer where they are usually overlaid by the use of fiduciary markers. This method of combining the two modalities can lead to registration errors as well as patient discomfort because both imaging procedures are time consuming. Collecting NMR and PET data in one scanner should solve the aforementioned problems. An additional benefit of obtaining PET images in a magnetic field (which is needed for NMR) is the anticipated increase in spatial resolution of a PET image. The physical limitations of intrinsic PET resolution are defined by positron range and angular deviation of annihilation photons which are generally assumed to be immutable. However it is possible that positron range can be limited in the presence of a strong magnetic field. Experimental proof of this claim will remove or significantly reduce one of the fundamental limitations of PET resolution. The specific aims of this investigation include: I. Modification of instrumentation so positron detector electronics are shielded from or made impervious to strong magnetic fields. Lightpipes will be used to couple the scintillation crystals to photomultiplier tubes that are shielded from the magnetic field. II. Measurement of positron range as a function of positron energy and magnetic field strength. A positron source will be moved past a pair of opposed well collimated photon detectors to detect coincidence photons as a function of position. The LSF (Line Spread Function) from this data will be used to determine positron range. The LSF should be reduced if positron range is significantly affected by a magnetic field. This research should lay the foundations for the development of combined NMR-PET scanner. The benefits of such a unit are enhanced resolution of PET images, correlation of PET and NMR data without the need for fiduciary markers, reduced cost (siting and staff) because both scans are acquired in the same unit and less patient discomfort.